Non-destructive inspections, which apply monitoring signals such as electricity, ultrasound wave, etc. to a machine or mount sensors to detect and compare normal signals to the detected signals, can be used for monitoring machine conditions.
Hereinafter, a prior art method of monitoring a welding deficiency will be described as an example.
Arc welding systems using robots or automatic welding devices are broadly used in the industrial field. In an arc welding operation, welding deficiencies such as lack of fusion, blow holes, etc. can be caused by pre-processing inferiority of a welding spot, non-homogeneity of a welding rod, insufficient supply of welding gas, etc.
Considering the arc welding operation, it is difficult for an operator to monitor the welding operations continuously. Further, it is hard to optically find a welding deficiency such as a blow hole formed in the welding spot. Thus, even if the welding deficiency has occurred, it may not be monitored perfectly. For example, in a car manufacturing system, a door panel or a body frame of the car is press-molded and parts with various forms and sizes are welded thereto by a welding robot. Then, they will be painted and assembled. In these continuous manufacturing processes, the welding deficiencies may not be monitored substantially. Also, as another example, in welding operations of a thick metal plate of heavy machinery for construction, a welding line is welded repeatedly. If a deficiency occurs in a first welding operation, then it is hard to detect the deficiency since the deficiency is usually examined optically after the welding operations are completed. In order to monitor the deficiencies of a machine which cannot be detected optically, non-destructive inspection such as ultrasonic wave inspection, X-ray inspection, etc. can be performed. However, the non-destructive inspection may require much time and money. Further, it is hard to use the non-destructive inspection for monitoring the welding spot of the machine with a complicated shape.
Commercial arc-welding monitoring method is performed by detecting the feed speed of a welding wire, welding current and voltage during the welding operation. For example, an arc-welding monitoring method of the prior art comprises the following steps: detecting signals of welding current and voltage and feed speed of the welding wire in a normal welding state; detecting signals of welding current and voltage and feed speed of the welding wire in a subject welding spot; comparing the signals detected at the subject welding spot to the signals of the normal welding state; and determining a section having deficiency where signals in the subject welding spot are different from the signals in the normal welding state. Deficiency occurrence signals can be determined by a change of minimum value or average value of the current or voltage at the welding spot, an abnormal peak point, abnormal peak points per unit time, change of feed speed of the welding wire and the like. They can be observed optically and compared to the normal state signals.
FIG. 1 is a block diagram showing an example of a prior art arc-welding monitoring device. As shown in FIG. 1, the prior art arc-welding monitoring device includes a first controller 11 for an arc-welding power source 10, a power converter 12, a first voltage sensor 13 and a first current sensor 14. The device further includes elements, which are mounted on the outside of the arc-welding power source 10, such as a second controller 23, a second voltage sensor 21 and a second current sensor 22. They are electrically connected at the arc-welding spot 30. Thus, the second voltage sensor 21 and the second current sensor 22 can detect the change of the current and the voltage during the arc-welding operation. Then, the deficiency occurrence can be determined by comparing the changes of the current and the voltage to those of the normal welding state.
However, in a real welding state, irregular and significant noises are often generated. Further, the signals may fluctuate when the subject welding spot is welded normally. Thus, the prior art method may not precisely detect a deficiency.